The objective of an anti-lock brake system (ABS) in a vehicle is to reduce the brake pressure at the different wheel brakes in case of excessive braking so that the wheels generate maximum brake force without locking. This aids in retaining vehicle stability and steerability while providing shorter stopping distances.
Typical anti-lock brake systems monitor the velocity at each of the wheels, decide whether the wheel is excessively slipping based on these velocity measurements, and modulate the braking pressure accordingly to avoid lock-up. Upon determining that a wheel is excessively slipping, the brake pressure is first reduced, maintained for a constant period of time, and finally increased until excessive wheel slip occurs again. The cycle of decreasing the brake pressure, maintaining constant brake pressure, and then increasing brake pressure is repeated until the anti-lock event ends. The frequency of this ABS cycling event typically occurs at about 3 Hz.
When the vehicle's drivetrain is engaged during an anti-lock braking event, brake torque is induced at the driven wheels by the drivetrain. When the ABS and the drivetrain braking occur simultaneously, there is an interaction or coupling that occurs. This interaction is most prevalent on low mu surfaces and has several undesirable effects. The first and most undesirable effect is a vibration in the drivetrain that is transferred to the vehicle and felt by the occupants. Another effect includes the introduction of excessive slip on the driven wheels which leads to instability in rear wheel drive vehicles or loss of steering control in front wheel drive vehicles. The overall effect is a roughness in ABS control which often leads to an increase in stopping distance.
The drivetrain vibration effect manifests itself as an oscillation in wheel speed that is at a higher frequency than the wheel speed oscillation induced by the cycling of the ABS. The drivetrain wheel speed oscillation frequency typically has a value that is more than twice the frequency of the ABS cycling. Therefore, a drivetrain-induced oscillation on the wheel speed typically has a frequency on the order of 8-10 Hz. The magnitude of the drivetrain oscillation is influenced by several factors including the ABS brake torque control. The magnitude decreases naturally with higher drive gears. Therefore, first and second gears show the largest magnitude, while the magnitude in third, fourth, and fifth gears is smaller.
One known prior art method of controlling the ABS upon detecting a drivetrain-induced oscillation includes delaying the pressure apply mode of the ABS cycle in order to dampen the wheel speed oscillations. A problem with this method is that if the oscillations are allowed to subside, then all indication of the drivetrain-induced oscillation will be lost. At this point, the system would revert to normal ABS control and eventually re-excite the drivetrain, causing the comfort and control problems to reappear. The system would once again have to detect the oscillations and revert to the modified control yielding a cyclic reoccurrence of the drivetrain influence.